Variable attenuators for RF signals are known. In one type of such an attenuator, a pi network includes variable impedance, voltage controlled PIN diodes in shunt input and output branches, as well as in a series branch. Generally, in the prior art circuits, a single current source supplies biasing current to all three diodes, to control the impedances of the branches. To achieve a linear db attenuation versus control voltage characteristic, in accordance with one prior art circuit, all of a linear input voltage swing is reduced to the range of the diode. Because a linear voltage across each of the diodes results in a logarithmic current variation, a linear db-voltage control is achieved. This prior art circuit reduces the normal control input voltage swing to less than one volt. Because of the relatively low voltage swing which can be applied to the diodes, the control voltage swing is at a level comparable to the amount of voltage change produced on the diodes as a function of temperature drift. Therefore, external temperature compensation circuits are generally employed.
In a second prior art circuit, wherein the diodes are driven by a single source, a piece-wise linear approximation generates antilogarithmic current characteristics as required. The use of a piece-wise linear driver to approximate an antilogarithmic function produces the desired antilogarithmic voltage output for a linearly controllable input. Each break point, however, must be precisely controlled and temperature tracked. Otherwise, the break points change as a function of temperature and proper tracking of the voltage source as a function of attenuation is generally not achieved.
In both these prior art circuits, a trade off must be made between linear operation and temperature control. Further, there is a tendency for input and output impedances of the attenuator to vary as a function of attenuation.
It is, accordingly, an object of the present invention to provide a new and improved variable RF attenuator.
Another object of the invention is to provide a new and improved variable RF pi type attenuator.
An additional object of the invention is to provide a new and improved RF attenuator having a linear db attenuation characteristic as a function of the level of a d.c. control voltage source.
An additional object of the present invention is to provide and new and improved variable RF attenuator having substantially constant characteristics over a relatively wide temperature range, e.g., minus forty degrees centigrade to plus seventy-five degrees centigrade.
An additional object of the invention is to provide a new and improved variable RF attenuator having relatively constant input and output impedance characteristics over a relatively wide range of attenuations.
An additional object of the invention is to provide a new and improved variable RF attenuator having a linear db attenuation characteristic as a function of the level of a d.c. control voltage source over a wide temperature range, and having relatively constant input and output impedances over the attenuation range.